Field of the Invention
The present invention relates to a magnesium casting alloy and a method of manufacturing such a magnesium casting alloy.
Related Art
Since magnesium is lighter than iron and aluminum, it is examined to use magnesium as a lightweight alternative material which replaces a member formed of an iron and steel material or an aluminum alloy material. As a magnesium alloy excellent in mechanical properties, casting and the like, AS91D is known.
However, in a general magnesium alloy, mechanical properties such as a tensile strength and creep elongation are lowered in a high-temperature region of about 200 to 250° C., and thus, it is impossible to obtain a high-temperature strength (tensile strength at a high temperature) comparable to a heat-resistant aluminum alloy such as an ADC 12 material or an A4032-T6 material.
Conventionally, as a commercially available magnesium alloy having heat resistance, WE54 is known in which a rare earth such as Y or a misch metal is added to enhance a high-temperature strength.
As a magnesium alloy having a high strength, for example, Patent Document 1 discloses a magnesium alloy which contains, with respect to the total amount, 1 to 4 atomic % of Zn, and 1 to 4.5 atomic % of Y, in which the remaining part is formed of Mg and an inevitable impurity and which is formed by casting and then extruding an Mg alloy where a composition ratio Zn/Y of Zn to Y falls within a range of 0.6 to 1.3. It is disclosed that this magnesium alloy includes an intermetallic compound Mg3Y2Zn3 and an Mg12YZn having a long-period structure and has both a high strength and a high ductility at room temperature.
Furthermore, a heat-resistant magnesium alloy is proposed which has a high strength under a high-temperature environment. For example, Non Patent Document 1 discloses that in an extruded material which is formed of an Mg95.8Zn2Y22Zr0.2 alloy, its proof stress (σ0.2) at 473K (200° C.) is 367 MPa.
Patent Document 2 discloses that in an extruded material which is formed of an Mg—Zn—Y alloy and which is obtained by extruding a cast product having a long-period multilayer structure phase, the hardness and the yield strength of the extruded material are enhanced as compared with the cast product (paragraph [0034]), and that in an extruded material of an Mg alloy formed of MgS97Zn1Y2, as a result of the measurements of a 0.2% proof stress, a tensile strength and an elongation at a test temperature of 200° C., a proof stress of 367 MPa is acquired (table 2).
Patent Document 3 discloses a heat-resistant magnesium alloy which contains 1 to 3 atomic % of Zn, 1 to 3 atomic % of Y and 0.01 to 0.5 atomic % of Zr and in which Zn/Y fails within a range of 0.6 to 1.3, in which an a-Mg phase and an intermetallic compound Mg3Y2Zn3 phase are minutely dispersed and in which a long-period multilayer structure phase is formed in the shape of a three-dimensional mesh. This Mg alloy is manufactured by casting it into a mold and cooling it at a rate of 10 to 103 K/second, and it is disclosed that the Mg alloy has both a high strength and a high ductility under a high-temperature environment of 200 to 250° C.    Patent Document 1: Japanese Patent No. 4500916    Patent Document 2: Japanese Patent No. 3905115    Patent Document 3: Japanese Unexamined Patent Application, Publication No. 2009-149352    Non Patent Document 1: Ienaga et al, “Casting Process and Mechanical of Large-Scale Extruded Mg—Zn—Y alloys”, SAE Technical Paper, 2013-01-0979, Apr. 8, 2013)    Non Patent Document 2: Kawamura Yoshihito, “Feature and future outlook of LPSO type magnesium alloy”, Materia, the Japan Institute of Metals, February 2015, Vol. 53, No. 2, p. 44-49